A major controversy exists in the endocrinology literature regarding the biosynthesis of the pro-androgen 11?- hydroxyandrostenedione (11?-OHAD). The synthesis of 11?-OHAD is thought mainly to be synthesized in the adrenal gland by 11?-hydroxylation of androstenedione (A4). Another route is through the side-chain cleavage of cortisol, although it is thought to be minor. Patients with 17-hydroxylase/C17,20-lyase (CYP17A1) deficiency fail to synthesize either cortisol or A4. When given cortisol exogenously, the urinary profile exhibits derivatives of 11?-OHAD. This suggests either a novel host enzyme, or a gut microbial enzyme responsible for side-chain cleavage of cortisol. We have identified genes in a highly active gut microbial pathway that may be responsible for the side-chain cleavage of cortisol. We hypothesize that gut microbiota are an important, and understudied component of the host endocrine system, which generate significant quantities of 11?-OHAD. This project will test this hypothesis by comparing stable isotope-labeled cortisol metabolism in germ-free pigs and pigs colonized with gut microbiomes of different complexity. We will determine effects on host colonic physiology through single- cell RNA-Seq, immunohistochemistry, and flow cytometry of immune cells. Understanding the physiological role of side-chain cleavage by gut bacteria is also important in determining future strategies to modulate pro- androgen formation. We will determine bacterial transcriptomic responses to cortisol and 11?-OHAD both in vitro and in vivo. Furthermore, to demonstrate causation between the genes responsible for cortisol side-chain cleavage (desAB) and host steroid metabolome profile, we will utilize a synthetic biology approach to engineer the desAB pathway (both wild type and inactive mutant) into E. coli and colonize the gnotobiotic pigs. These studies are expected to resolve an enigma that has existed for decades, and may result in a paradigm-shift if it can be shown that the gut microbiota contributes significantly to a quantitatively major host steroid and pro- androgen.